1. Field of the Invention
The present invention is directed to transferring data between a packet-switched network and a line-switched network.
2. Description of the Related Art
In contemporary communications systems, connections to transfer continuous data streams, e.g., for voice or video communications, are also routed to an increasing extent via packet-switched communications networks, such as LANs (Local Area Network), MANs (Metropolitan Area Network) or WANs (Wide Area Network). Internet telephony, which is also frequently referred to as “Voice over Internet Protocol” (VoIP), for example, is based on this technology.
To transmit a continuous data stream via a packet-switched communications network, the data stream is divided up into individual data packets, which are in each case provided with a destination address and are transmitted via the packet-switched communications network. On emerging from the packet-switched communications network, a continuous data stream is recomposed from the data packets. Since the data packets are transmitted in the packet-switched communications network essentially independently from one another, the data packets do not normally arrive at equidistant time intervals at the exit. To equalize transit time fluctuations of this type, the data packets are temporarily stored, prior to the composition of the data stream, in a data-packet buffer memory operating according to the continuity principle, from which they are read out at constant time intervals. A continuous data stream can be reconstructed in this way from data packets arriving at irregular time intervals. A data-packet buffer memory of this type is also frequently referred to as a jitter buffer. However, a disadvantage of a jitter buffer of this type is that the data transmission is thereby additionally delayed.
Furthermore, in the transmission of user data within data packets, a higher transmission bandwidth is usually required than for the transmission of these user data via a line-switched network, since a data packet header with address and control data must be additionally transmitted with each data packet. For this reason, inter alia, a data stream which is to be transmitted is frequently compressed using data-compression methods in the transfer into a packet-switched communications network. In the transfer from a packet-switched communications network into a line-switched communications network, the data stream is frequently decompressed by the gateway, since, for example, ISDN terminals cannot usually carry out decompression. However, with many conventional compression methods, e.g., according to ITU-T Recommendation G.723, information loss occurs.
Insofar as the connection between the sender and recipient of a data stream is routed via an individual, continuous packet-switched communications network, a delay which occurs in the conversion of the data stream or a slight information loss is usually tolerable. However, particularly in the case of modern heterogeneous communications networks, a multiple transfer frequently occurs between one or more packet-switched communications networks and one or more line-switched communications networks between the sender and recipient of a data stream. However, as a result of the repeated conversion of the data which are to be transmitted at the transfer points between the communications networks, the delay times and information losses at the individual transfer points are added together. In the case of a transmission of voice data, even a twofold compression and decompression of the voice data according to the aforementioned G.723 Recommendation results in the voice output at the receiving end being barely comprehensible. In addition, a long transmission delay in a voice transmission is perceived by a user as very negative.